In general, a driver circuit uses a controller for controlling light-emitting elements. In order to drive the controller, the driver circuit supplies power to the controller. The driver circuit may internally include a power supply circuit or use an external power source for power supply.
FIG. 1 is a circuit diagram that illustrates a typical driver circuit having a buck structure. Particularly, FIG. 1 illustrates an LED lamp driver circuit as an example of a driver circuit.
As illustrated in FIG. 1, the buck structure is a step-down structure. The driver circuit includes an inductor L 1, a freewheeling diode DFW 2, a power switch MSW 3, light emitting diodes LEDs 4, a current sensing resistor Rcs 5, a control circuit 6, a switching element 7, a resistor 8, a Zener diode DZI 9, and a VDD capacitor CVDD 10. The inductor 1, the freewheeling diode 7, and the LEDs 4 form a closed loop.
When the power switch 3 is turned on, current flows through the LEDs 4 and the LEDs 4 are turned on. When the power switch 3 is turned off, the current circulates in the closed loop formed of the inductor 1, the freewheeling diode 2, and the LEDs 4. The current sensing resistor 5 detects current flowing through the inductor 1 and the LEDs 4 when the power switch 3 is turned on. The current sensing resistor 5 is also connected to the control circuit 6 to control current flowing in the LEDs 4. The driver circuit necessarily includes an integrated circuit (IC) such as the control circuit 6 for driving the buck structure. The driver circuit requires a separate power source for driving the control circuit.
The high-voltage switching element 7 supplies or interrupts power to the control circuit 6 depending on switching operation thereof. The high-voltage switching element 7 implements a linear regulator that supplies voltage V2 to the control circuit 6 in proportional to voltage V1 supplied to the Zener diode 9. The VDD capacitor 10 is disposed between a power input terminal VDD of the control circuit 6 and ground. The VDD capacitor 10 removes noise.
As illustrated in FIG. 1, the driver circuit has a simple circuit structure that efficiently supplies power to the control circuit 6 depending on switching operation of the high-voltage switching element 7. The driver circuit, however, has a disadvantage of high power consumption, especially when power source voltage Vsup is high. The driver circuit also has a disadvantage of high manufacturing cost due to expensive high-voltage switching element 7.
FIG. 2 is a circuit diagram that illustrates a typical driver circuit having a transformer instead of an inductor. Unlike the driver circuit of FIG. 1, the driver circuit of FIG. 2 includes a transformer 10 instead of an inductor. The transformer 10 such as an X-former can generate power without requiring a high voltage switching element M1.
As illustrated in FIG. 2, the transformer 10 generates power and supplies it to a control circuit 60. When a power switch MSW 30 is turned off, current circulates in a closed loop formed of the transformer 10, a freewheeling diode DFW 20, and LEDs 40. The current turns on the freewheeling diode 20. Accordingly, a primary voltage V11 of the transformer 10 becomes approximately equal to a forward voltage VF of the LEDs 40.
The transformer 10 also generates a secondary voltage V12 having a voltage level corresponding to the turns ratio of the primary voltage V11. For example, the second voltage V12 is about N times lower than the primary voltage V11 when the turns ratio of the transformer 10 is N:1, where N is a natural number. The secondary voltage V12 of the transformer 10 causes current to flow through a diode DM 70 and charges a VDD capacitor CVDD 80, thereby supplying power to the control circuit 60.
The driver circuit of FIG. 2 has advantages of relatively low manufacturing cost and low power consumption as compared with the driver circuit of FIG. 2. The driver circuit of FIG. 2, however, has disadvantages as well. For instance, the driver circuit of FIG. 2 requires different types of transformers depending on the number of the LEDs 40. It is because a secondary voltage V12 is needed to supply power to the control circuit 60 and the second voltage V12 changes depending on the forward voltage VF of the LEDs 40. For example, if the number of the LEDs 40 changes then the load of the LEDs 40 changes. Such load variation causes changing the forward voltage VF of the LEDs 40. The driver circuit of FIG. 2, therefore, requires a transformer 10 having a different turns-ratio corresponding to the number of the LEDs 40.
As described above, the driver circuit of FIG. 2 requires a transformer having a different turns-ratio corresponding to the number of LEDs. Such a transformer has been manufactured through a complicated process as compared to the inductor. Accordingly, including a transformer could be disadvantage of the driver circuit of FIG. 2. In addition, it is required to appropriately set magnetizing inductance at the primary side of a transformer when the transformer is manufactured. If not, it could cause a problem in operation of a buck structure.